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The range extends about 40 mi (64 km) east–west from the Hollywood Hills in Los Angeles to Point Mugu in Ventura County. The mountains form a line between the San Fernando Valley and the Los Angeles Basin, separating "the Valley" on the north and west-central Los Angeles on the south. The Santa Monica Mountains are parallel to Santa Susana Mountains, on the north side the San Fernando Valley. Beginning at the eastern end of the San Fernando Valley, the mountains are bordered to the north by the Los Angeles River. The river flows south after Elysian Park, defining the easternmost edge of the mountains. Mt. Washington, on the other side of the river, has almost the same native plants and weather.

Scientists who study rocks say that the northern Channel Islands are a westward extension of the Santa Monicas into the Pacific Ocean. The range was created by the Raymond Fault which made the rocks push up and sink down over time. This has made old volcano rock come to the surface. Malibu Creek cuts the mountain range in two.

The Santa Monica Mountains have dry, warm to humid summers and wet, mild to cool winters. In the summer, the weather is quite dry, which makes the range prone to wildfires. Snow is unusual in the Santa Monica Mountains, since they are not as high as the nearby San Gabriel Mountains.

On January 17, 2007, an unusually cold storm brought snow in the Santa Monica Mountains. Malibu picked up three inches (eight centimeters) of snow – the first measurable snow in five decades (50 years). Snow was reported on Boney Peak, in the winter of 2005; and in March 2006, snow also fell on the summit of the mountain.

The southwestern headwaters of the Los Angeles River are in the Santa Monica Mountains. The Los Angeles River also forms the northern boundary of the mountains from the easternmost part of the San Fernando Valley to Elysian Park, where the river turns south, thereby defining the easternmost part of the mountain range.

The Santa Monica Mountains are in the California chaparral and woodlands ecoregion and are covered by hundreds of local plant species, some of which are very rare, and others of which have become popular ornamentals. The range is host to an immense variety of wildlife, from mountain lions to the endangered steelhead. The mountain lion population within the Santa Monica Mountains (which includes the Simi Hills & Santa Susana Pass) is severely depleted with only seven known living adult individuals. The primary cause of the decline is due to a combination of traffic related mortality (three from the area were killed within a matter of months), anti-coagulants ingested from human-poisoned prey (two individuals within the Simi Hills) and attacks by other, more dominant mountain lions (an elder male, known as P1, killed both his son and his mate. This is thought to be due to a lack of space available.) Snakes are common but only occasionally seen – the Southern Pacific Rattlesnake (the only venomous species), Mountain Kingsnake, California Kingsnake, Gopher snake, and Garter snake.